Normally, signal receivers and particularly television signal receivers employ an automatic frequency control (AFC) circuit wherein a tuner stage includes the usual mixer and oscillator stage and a received signal is heterodyned to provide a signal at a so-called intermediate frequency or IF signal. This IF signal is applied to a group of IF amplifier stages and to a discriminator tuned to a given frequency. Any deviation in the signal from the tuned frequency provides an error or AFC correction signal. In turn, the error signal is coupled back to the oscillator of the tuner in a manner such that the oscillator tuning is altered to reduce the frequency deviation of the signal applied to the IF stages with respect to the tuned frequency of the discriminator stages.
Although the above-mentioned AFC circuitry has been and still is utilized in numerous applications with a great deal of success, it has been found that the presettability of many present-day tuners and particularly 70 detent tuners is frequently insufficient when such circuitry is employed. More specifically, it has been found that AFC systems employing a discriminator tend to provide an error signal having a "pull-in" range limited to a small portion of the discriminator response curve.
For example, apparatus having an adjacent carrier signal, such as the sound carrier in a TV receiver, provides an error correction response which tends to push the oscillator of the tuner stage away from rather than toward the desired tuning when the discriminator response curve has a negative-going slope. Also, apparatus employing a DC amplifier for the error voltage does not always have an amplifier bias voltage and an error signal quiescent voltage which coincide. Thus, the system will tend to "block" due to this non-coincident operation condition when the discriminator response is a negative-going slope and is of a magnitude less than the difference between the above-mentioned bias and quiescent potentials.
As a result, normal AFC systems utilizing discriminator type circuitry do not normally provide a wide band "pull-in" range. Moreover, wide band "pull-in" is a highly desirable condition in AFC systems when tuners, seventy detent or others, with limited presettability capabilities are employed.